-
124 Engineering Volume 1 · Issue 1 · March 2015
www.engineering.org.cn
Engineering 2015, 1(1): 124–130DOI 10.15302/J-ENG-2015013
Dual-Material Electron Beam Selective Melting: Hardware
Development and Validation StudiesChao Guo1, 2, 3#, Wenjun Ge1, 2,
3#, Feng Lin1, 2, 3*
1 Department of Mechanical Engineering, Tsinghua University,
Beijing 100084, China; 2 Key Laboratory for Advanced Materials
Processing Technology (Ministry of Education of China), Tsinghua
University, Beijing 100084, China; 3 Biomanufacturing and Rapid
Forming Technology Key Laboratory of Beijing, Tsinghua University,
Beijing 100084, China* Correspondence author. E-mail:
[email protected]# These authors contributed equally to this
work.Received 16 February 2015; received in revised form 25 March
2015; accepted 25 March 2015
© The Author(s) 2015. Published by Engineering Sciences Press.
This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/)
3D Printing—Article
ABSTRACT Electron beam selective melting (EBSM) is an additive
manufacturing technique that directly fabricates three-dimensional
parts in a layerwise fashion by using an electron beam to scan and
melt metal powder. In recent years, EBSM has been successfully used
in the additive manufacturing of a variety of materials. Previous
research focused on the EBSM process of a single material. In this
study, a novel EBSM process capable of building a gradient
structure with dual metal materials was developed, and a
powder-supplying method based on vibration was put forward. Two
different powders can be supplied individually and then mixed. Two
materials were used in this study: Ti6Al4V powder and Ti47Al2Cr2Nb
powder. Ti6Al4V has excellent strength and plasticity at room
temperature, while Ti47Al2Cr2Nb has excellent performance at high
temperature, but is very brittle. A Ti6Al4V/Ti47Al2Cr2Nb gradient
material was successfully fabricated by the developed system. The
microstructures and chemical compositions were characterized by
optical microscopy, scanning microscopy, and electron microprobe
analysis. Results showed that the interface thickness was about 300
μm. The interface was free of cracks, and the chemical compositions
exhibited a staircase-like change within the interface.
KEYWORDS additive manufacturing, electron beam, selective
melting, gradient materials, titanium alloy, TiAl alloy
1 IntroductionElectron beam selective melting (EBSM) is an
additive manufacturing (AM) process that utilizes an electron beam
to fabricate three-dimensional objects layer by layer, based on a
powder bed. In the EBSM process, the electron beam first preheats
the substrate to a high temperature. Next, the building platform is
lowered by an amount equal to the layer thickness, and a powder
layer is spread onto the substrate.
For each powder layer, the process includes two steps: ①
pre-heating the powder layer and increasing the build tempera-ture;
and ② melting the cross-section. As the power density of an
electron beam is extremely high, and as almost all of the electron
beam energy can be absorbed by the metal mate-rials, the EBSM
process is favorable for manufacturing fully dense parts with high
melting-point materials, such as Ti al-loys. In addition, the build
temperature during the process can be maintained at a high level
(above 700 ℃), decreasing the thermal stress of built parts.
In recent years, EBSM has been successfully used in the AM of a
variety of materials, including 316L stainless steel, Ti6Al4V,
copper (Cu), Inconel 625 superalloy, cobalt (Co)-based superalloy,
and TiAl alloy [1–5].
Previous research mainly focused on the EBSM of a single
material. However, the application of EBSM requires further
expansion, to allow this technology to be used to its full
po-tential and capacity. One of the possible new applications of
EBSM is the manufacturing of functional gradient materi-als (FGMs).
FGMs were first proposed in the late 1980s. The main purpose of
manufacturing FGMs is to optimize the distribution of mechanical
properties in order to meet the re-quirements of complex working
conditions. FGMs have been widely used in aerospace industries and
in biological fields, where they are extremely important
materials.
Many manufacturing methods have been employed to produce
high-quality FGMs. These methods include powder metallurgy (PM),
self-propagation high-temperature synthe-sis (SHS), plasma
spraying, and AM technology [6]. In recent years, AM has been
considered to be a promising manu-facturing technology for
fabricating FGMs. Banerjee et al. fabricated Ti8AlxV gradient
materials using the laser metal deposition (LMD) process, in which
the fraction of vanadium (V) gradually varied from 0 to 25% [7].
Sahasrabudhe et al. investigated the interface of a stainless steel
and Ti6Al4V gradient material fabricated by the LMD process [8].
Wang
Research
-
125www.engineering.org.cn Volume 1 · Issue 1 · March 2015
Engineering
3D Printing—Article ResearchResearch
et al. manufactured Ti/Ti6Al2ZrMoV and Ti6Al2ZrMoV/Ti47Al2.5VCr
gradient materials by LMD, and conducted detailed research on the
chemical compositions, microstruc-tures, and mechanical pro perties
of the gradient materials [9, 10]. Liu et al. fabricated a gradient
material of stainless steel and copper by selective laser melting
(SLM) [11].
Although AM methods were used to fabricate these FGMs, the
researchers mainly focused on laser processes, such as LMD and SLM.
However, there are two issues involved in the laser-based AM of
gradient materials. First, the absorption of laser energy by
different materials varies considerably. Therefore, as the material
composition changes, the laser power needs to be adjusted in real
time to maintain a consis-tent melting condition. The adjustment of
the laser power in-creases the difficulty of process control.
Second, the thermal stress during the laser process is high,
resulting in cracks in the built parts, especially at the interface
of two different ma-terials. In Refs. [8] and [11], cracks were
found at the interface of the gradient materials. Heat treatment
such as high tem-perature aging is essential for laser-built parts,
to decrease or eliminate the residual thermal stress [10].
The EBSM process is a promising technology for building gradient
materials of better quality. The differences in ab-sorption of
electron beam energy between different materials are small, and the
higher build temperature during EBSM re-duces the risk of thermal
stress cracks. However, no research on the EBSM of gradient
materials has been reported to date. In this study, a novel EBSM
process involving dual materials was developed. Two different
powders were used to fabricate parts. The mixing ratio of each
powder layer can be tailored, so that parts can be fabricated with
a material that undergoes a gradual change. A Ti6Al4V/Ti47Al2Cr2Nb
gradient mate-rial was successfully fabricated using EBSM in this
study, and the microstructures and chemical compositions of the
fabricated samples were analyzed.
2 Hardware developmentAn EBSM system with the capacity for
dual-material process-ing was developed, and a powder-supplying
method based on vibration was put forward. Two different powders
can be supplied individually and then mixed. In order to prolong
the spreading comb’s lifetime and avoid comb-tooth breakage, a
comb-inclined powder-spreading method was employed.
2.1 Powder-supplying method based on vibrationVibration was used
as the power source to supply the powder materials through contact
friction. Figure 1 shows the prin-
cipal model of the powder-supplying method. In the original
state, as shown in Figure 1(a), a powder exit is formed be-tween
the vibration plate and the powder storage container. The powder
flows out of the exit, keeping still on the vibra-tion plate
because of the balance between gravity and fric-tion. In Figure
1(b), the vibration plate moves forward with acceleration a. The
powder inside the storage container re-mains still, due to the
powder pressure and the storage wall restriction. The powder
outside the storage container moves along the vibration plate, as
long as the internal friction co-efficient of the powder and the
static friction coefficient be-tween the powder and the vibration
plate are above a/g (where g is the gravity acceleration). The
moving distance equals the vibration amplitude, A. The motion of
the powder breaks the balance, so as a result, powder flows out of
the storage container, re-establishing a balanced state as shown in
Figure 1(c). In Figure 1(d), the vibration plate moves back, and
the powder remains still because of the restriction of the storage
wall. Therefore, the powder on the vibration plate moves a distance
of A in a vibration cycle. If the vibration plate keeps working,
the powder will continuously move on the vibra-tion plate,
generating a stable powder flow off the vibration plate. The
powder-supplying rate increases with an increase of vibration
amplitude, powder exit height, and the powder materials’
flowability.
The powder supplier has no revolute pair, and thus is high-ly
reliable in a dusty environment, and does not get stuck by powder
particles. The powder-supplying rate can be changed by altering the
vibration amplitude and the height of the powder exit.
2.2 Dual powders mixing in tailored proportionFigure 2 is a
schematic of the dual powders mixer. Two pow-der suppliers based on
vibration are placed side by side, facing each other. A
powder-mixing box is placed directly below the edges of the
vibration plates, and a weight sensor is used to measure the weight
of powder in the mixing box in real time. Once the powder weight
reaches the desired value, the vibrator stops working. The two
powders can be supplied into the mixing box individually and
precisely in order to ob-tain a mixture with a tailored proportion.
Next, as shown in Figure 3, the mixing box rotates back and forth,
blending the two powders, and finally pours the mixture onto the
working platform. The powder spreader pushes the powder onto the
building tank, filling the space on the deposited layers.
Assuming that the powder layer thickness is Δh, the area of the
building tank is S, and the volume fraction of powder A in the
layer is εA, then the amount of powder A, mA, and the
Figure 1. The schematics of the powder-supplying method. (a)
Original state; (b) vibration plate moves forward, breaking the
balance; (c) powder flows, re-balancing; (d) vibration plate moves
back.
-
126 Engineering Volume 1 · Issue 1 · March 2015
www.engineering.org.cn
3D Printing—ArticleResearch
amount of powder B, mB, can be calculated via Eqs. (1) and (2):
mA = kρAΔhSεA (1) mB = kρBΔhS(1–εA) (2)
where k > 1, to compensate for the powder waste outside the
building tank; ρA and ρB are the apparent densities of pow-ders A
and B; and for a process involving a single material, εA is set at
0 or 1.
2.3 Comb-inclined powder spreadingDuring EBSM, balling phenomena
often occur, resulting in bumps on the deposited surface. When this
happens, the powder spreader must scrape over the bump, as shown in
Figure 4, which causes the comb teeth to bend. Vertical combs are
commonly used in AM technologies that are based on a powder bed.
Unfortunately, permanent deformation or breaking of the comb teeth
often occurs. After scraping over the balling surface several
times, irrecoverable deformation is found in some of the comb
teeth. A deformed comb causes an
uneven surface of the powder bed, and worsens the fabrica-tion
quality in the following deposition. In this study, instead of
using a vertical comb, an inclined comb was utilized in order to
reduce the risk of tooth deformation or breakage.
The maximum von Mises stress on a comb tooth scraping over a
bump was computed by a finite element model, and the results are
plotted in Figure 5. As the inclined angle increases, the maximum
von Mises stress on the comb decreases greatly. For example, if the
bump height is 0.3 mm, the maximum von Mises stress on a comb at a
25° inclined angle is only 20% of the stress on a vertical comb. A
lower stress level means a lower risk of breakage for the comb
tooth, and thus efficiently extends the comb lifetime.Figure 2.
Supplying the two powders.
Figure 3. Mixing the two powders.
Figure 4. Vertical and inclined combs.
3 Validation experimentsTwo powders were used in this study:
Ti6Al4V and Ti47Al-2Cr2Nb powder. These two powders were both
gas-atomized and produced by the Northwest Institute for
Non-Ferrous Metal Research, China. The size distribution of the
Ti6Al4V powder was 50–260 μm, and the average particle size was
about 125 μm. The chemical compositions (in weight percent-ages)
were 6.46% aluminum (Al), 4.13% V, and the balance titanium (Ti).
The size distribution of the Ti47Al2Cr2Nb powder was 40–150 μm. The
Ti47Al2Cr2Nb powder had a chemical composition (in atomic
percentages) of 46.51% Al, 0.02% niobium (Nb), 0.02% chromium (Cr),
and the balance Ti. Ti6Al4V has excellent strength and plasticity
at room temperature, while Ti47Al2Cr2Nb has excellent performance
at high temperatures, but is very brittle. It is difficult for
tra-ditional processes to combine the plastic Ti6Al4V with the
brittle Ti47Al2Cr2Nb.
Experiments were conducted on a team-developed EBSM system,
shown in Figure 6. The maximum power of the elec-tron beam is 3 kW
(with an acceleration voltage of 60 kV and a maximum beam current
of 50 mA). The previously described methods of a vibration-based
supply, dual powders mixing, and comb-inclined spreading were
applied to this system. The system contains two exchangeable
building tanks with sizes of 100 mm × 100 mm × 100 mm and 250 mm ×
250 mm × 250 mm. In this study, the smaller building tank was used
to
Figure 5. Maximum von Mises stress of comb tooth at different
inclined angles.
-
127www.engineering.org.cn Volume 1 · Issue 1 · March 2015
Engineering
3D Printing—Article ResearchResearch
fabricate small square samples.A 316 L stainless steel substrate
with a size of 90 mm × 90 mm
× 10 mm was placed in the powder bed. Before depositing, the
electron beam scanned the substrate for 20 min with a beam current
of 15 mA, a defocusing current of 150 mA, a scanning line spacing
of 1 mm, and a scanning velocity of 10 m.s–1. The layer thickness
in this study was set at 100 μm. After one layer of powder was
applied, the process included two steps: ① pre-heating the powder
bed before melting; and ② melting through the powder layer and part
of the previous layers. After fabrica-tion, the samples remained in
the vacuum chamber for approxi-mately 4–5 h to cool down to room
temperature.
The fabricated samples were cut along the deposition di-rection
using electron-discharge machining, and were then mounted, ground,
polished, and etched. Kroll’s reagent was used to etch the samples.
The microstructures of the samples were characterized by optical
microscope (OM), scanning electron microscope (SEM) with
backscattered electron (BSE) mode, X-ray diffraction (XRD), and
transmission electron mi-croscope (TEM).
4 Results and discussionThe square Ti6Al4V samples built by EBSM
are shown in Figure 7. The cross-section sizes of these samples are
about 20 mm × 20 mm. In addition to Ti6Al4V, samples of
Ti47Al-2Cr2Nb and Ti6Al4V/Ti47Al2Cr2Nb gradient materials were also
fabricated. The analyses of their microstructures will be described
later in this section.
4.1 Microstructures of Ti6Al4V samplesFigure 8 shows the
microstructures of the Ti6Al4V fabricated by EBSM. As shown in
Figure 8(a), columnar crystals paral-lel to the building direction
can be seen. Within a distance of about 1 mm from the top, the
microstructures were α′-martensite plates, as shown in Figure 8(b).
Within the rest of the section, the microstructures were
basket-weave struc-
tures with acicular α-phase grains (black) surrounded by the
interfacial β-phase (white).
In their study, Hrabe and Quinn found that the micro-structures
did not vary with distance from the build plate [12]. In this
study, however, a change of microstructures was observed.
α′-martensite exists within a small distance from the top, and the
α/β phase exists within the rest. This change provides evidence of
the phase transformation during EBSM: For a given layer, the liquid
phase transforms into the β phase several times because the beam
penetration depth is greater than the layer thickness. Because of
the very high cooling rate, the β phase first transforms into
α′-martensite. Next, the α′-martensite decomposes into the α/β
phase, because the layer is heated to the phase-transition
temperature in the con-tinued building cycle. Figure 9 shows the
XRD patterns of the
Figure 6. The dual-material EBSM system. (a) The system at a
glance; (b) devices in the chamber.
Figure 7. Square Ti6Al4V samples built by EBSM.
-
128 Engineering Volume 1 · Issue 1 · March 2015
www.engineering.org.cn
3D Printing—ArticleResearch
Ti6Al4V, which confirms the existence of the α and β phase. The
residual α′-martensite within the top region in this study may
relate to a lack of thermal insulation of the chamber. After
building, the top region of the part quickly cools down and does
not have enough time to finish decomposing from α′-martensite to
the α/β phase.
4.2 Microstructures of Ti47Al2Cr2Nb samplesFigure 10 shows the
BSE microstructures of the Ti47Al2Cr2Nb fabricated by EBSM.
Dendritic morphology can be found in the top region of the sample.
The angle between the main dendrite arm and the secondary dendrite
arm is 90°, which proves that the primary solidification phase is
the β phase. As seen in Figure 10, β dendrites grow along the
building direction. According to the phase diagram, the
solidification process is shown as follows:
L → L + β → β → β + α + γ → α → α + γ → α2 + γ
Figure 8. Microstructures of Ti6Al4V fabricated by EBSM. (a)
Lower magnification by OM; (b) SEM picture of α′-martensite near
the top; (c) SEM picture of the α/β phase away from the top.
Figure 9. XRD pattern of EBSM-fabricated Ti6Al4V.
Figure 10. BSE microstructures of Ti47Al2Cr2Nb fabricated by
EBSM.
The XRD patterns of the materials confirm that the main phase is
γ and α2, as shown in Figure 11. The grain size of the alloys with
fully lamellar microstructure is 5–20 μm, which is considerably
smaller than the typical grain in an as-cast lamellar structure
(Figure 12). Figure 13 shows a TEM image of the full lamellar
structure of the EBSM-fabricated Ti47Al2Cr2Nb. It can be seen that
the thickness of the γ laths is about 0.5 μm and that of the α2
laths is 0.1 μm. The γ slices and α2 slices are arranged
alternately.
Figure 11. XRD pattern of EBSM-fabricated Ti47Al2Cr2Nb.
Figure 12. BSE image of EBSM-fabricated Ti47Al2Cr2Nb.
-
129www.engineering.org.cn Volume 1 · Issue 1 · March 2015
Engineering
3D Printing—Article ResearchResearch
A linear scan analysis using an electron probe micro- analyzer
(EPMA) was employed to test the composition
Figure 13. TEM image of the lamellar structure of the
EBSM-fabricated Ti47Al2Cr2Nb.
4.3 Ti6Al4V/Ti47Al2Cr2Nb gradient structuresA
Ti6Al4V/Ti47Al2Cr2Nb gradient material was successfully fabricated
by EBSM. The material in the bottom 10 layers was Ti47Al2Cr2Nb, and
the material in the top 20 layers was Ti6Al4V. Figure 14 shows the
macrograph of the vertical sec-tion of the gradient material. No
cracks were found in the interface.
Figure 14. Macrograph of the vertical section of the gradient
material.
Figure 15. Composition change in the building direction of the
sample.
Figure 16 shows the microstructures in a different region of the
Ti6Al4V/Ti47Al2Cr2Nb gradient structure. Results show that the
fully lamellar microstructure consisted of α2-Ti3Al, and that
γ-TiAl was formed on the Ti47Al2Cr2Nb side, while a coarse
basket-weave microstructure was formed on the Ti6Al4V side. The α
phase thickness in the basket-weave microstructure was about 2 μm.
In the top 5 layers at the Ti6Al4V side, the microstructure was
needle-like α′ lath mar-tensite with a width of about 1 μm.
5 ConclusionsIn this paper, a novel EBSM process capable of
building a gradient structure with dual metal powders was
developed. A powder-supplying method based on vibration was put
forward. In this process, two different powders can be sup-plied
individually and then mixed. In order to avoid yielding or breakage
of the comb tooth, a low-deformation powder-
change in the building direction of the sample, and the results
are shown in Figure 15. The results show that the interface
thickness was about 300 μm. In the interface, the composition
varied from Ti47Al2Cr2Nb to Ti6Al4V. Instead of a linear step
change, two waves can be found in the con-tent change curves of Ti
and Al. The wave length is about 100 μm, which exactly equals the
layer thickness. The presence of these waves demonstrates that the
previous several layers were re-melted in the melting of the new
layer, resulting in the staircase-like change.
Figure 16. Microstructures of the gradient structure. (a) Top
region of the Ti6Al4V side; (b) main region of the Ti6Al4V side;
(c) Ti47Al2Cr2Nb side.
-
130 Engineering Volume 1 · Issue 1 · March 2015
www.engineering.org.cn
3D Printing—ArticleResearch
spreading device was designed.Single-material samples were
fabricated with Ti6Al4V and
Ti47Al2Cr2Nb, respectively. For the Ti6Al4V, the
microstruc-tures were dominantly α/β phase, and α′-martensite was
found within a small distance from the top. For the Ti47Al-2Cr2Nb,
the microstructures were fully lamellar microstruc-tures consisting
of α2-Ti3Al and γ-TiAl.
A Ti6Al4V/Ti47Al2Cr2Nb gradient material was success-fully
fabricated. The interface thickness was about 300 μm. The interface
was free of cracks, and the Ti element and Al el-ement exhibited a
staircase-like change within the interface.
Acknowledgements The authors would like to acknowledge the
funding of 2013 Beijing Science and Technology Development Project
(D13110400300000 and D131100003013002).
Compliance with ethics guidelinesChao Guo, Wenjun Ge, and Feng
Lin declare that they have no conflict of interest or financial
conflicts to disclose.
References1. Y. N. Yan, H. B. Qi, F. Lin, W. He, H. R. Zhang, R.
J. Zhang. Produced three-
dimensional metal parts by electron beam selective melting.
Chin. J. Mech.
Eng., 2007, 43(6): 87–92 (in Chinese)
2. D. Cormier, O. L. A. Harrysson, T. Mahale, H. A. West.
Freeform fabrica-
tion of titanium aluminide via electron beam melting using
prealloyed
and blended powders. Adv. Mater. Sci. Eng., 2008, 2007:
6822–6825
3. L. E. Murr, et al. Metal fabrication by additive
manufacturing using laser
and electron beam melting technologies. J. Mater. Sci. Technol.,
2012, 28(1):
1–14
4. L. E. Murr, et al. Microstructures of Rene 142 nickel-based
superalloy fab-
ricated by electron beam melting. Acta Mater., 2013, 61(11):
4289–4296
5. S. H. Sun, Y. Koizumi, S. Kurosu, Y. P. Li, H. Matsumoto, A.
Chiba. Build
direction dependence of microstructure and high-temperature
tensile
property of Co-Cr-Mo alloy fabricated by electron beam melting.
Acta Ma-
ter., 2014, 64: 154–168
6. Y. Chen, C. Zeng, M. Yan. Research process of Ti base
functional gradient
materials. Mater. Rev., 2012, 26(S1): 267–270 (in Chinese)
7. R. Banerjee, D. Bhattacharyya, P. C. Collins, G. B.
Viswanathan, H. L. Fra-
ser. Precipitation of grain boundary a in a laser deposited
compositionally
graded Ti-8Al-xV alloy—An orientation microscopy study. Acta
Mater.,
2004, 52(2): 377–385
8. H. Sahasrabudhe, R. Harrison, C. Carpenter, A. Bandyopadhyay.
Stainless
steel to titanium bimetallic structure using LENSTM. Addit.
Manuf., 2015, 5:
1–8
9. Y. Liang, X. Tian, Y. Zhu, J. Li, H. Wang. Compositional
variation and
microstructural evolution in laser additive manufactured
Ti/Ti-6Al-2Zr-
1Mo-1V graded structural material. Mater. Sci. Eng. A, 2014,
599: 242–246
10. H. P. Qu, P. Li, S. Q. Zhang, A. Li, H. M. Wang.
Microstructure and me-
chanical property of laser melting deposition (LMD) Ti/TiAl
structural
gradient material. Mater. Des., 2010, 31(1): 574–582
11. Z. H. Liu, D. Q. Zhang, S. L. Sing, C. K. Chua, L. E. Loh.
Interfacial charac-
terization of SLM parts in multi-material processing:
Metallurgical diffu-
sion between 316L stainless steel and C18400 copper alloy.
Mater. Charact.,
2014, 94: 116–125
12. N. Hrabe, T. Quinn. Effects of processing on microstructure
and mechani-
cal properties of a titanium alloy (Ti-6Al-4V) fabricated using
electron
beam melting (EBM), part 1: Distance from build plate and part
size. Ma-
ter. Sci. Eng. A, 2013, 573: 264–270
Dual-Material Electron Beam Selective Melting: Hardware
Development and Validation Studies1 Introduction2 Hardware
development2.1 Powder-supplying method based on vibration2.2 Dual
powders mixing in tailored proportion2.3 Comb-inclined powder
spreading
3 Validation experiments4 Results and discussion4.1
Microstructures of Ti6Al4V samples4.2 Microstructures of
Ti47Al2Cr2Nb samples4.3 Ti6Al4V/Ti47Al2Cr2Nb gradient
structures
5 ConclusionsAcknowledgementsCompliance with ethics
guidelinesReferences